U.S. patent application number 14/096911 was filed with the patent office on 2014-06-12 for method and apparatus for high speed acquisition of moving images using pulsed illumination.
This patent application is currently assigned to KLA-Tencor Corporation. The applicant listed for this patent is KLA-Tencor Corporation. Invention is credited to David L. Brown, Yung-Ho Chuang, Yury Yuditsky.
Application Number | 20140158864 14/096911 |
Document ID | / |
Family ID | 50879908 |
Filed Date | 2014-06-12 |
United States Patent
Application |
20140158864 |
Kind Code |
A1 |
Brown; David L. ; et
al. |
June 12, 2014 |
Method And Apparatus For High Speed Acquisition Of Moving Images
Using Pulsed Illumination
Abstract
A method of operating an image sensor with a continuously moving
object is described. In this method, a timed delay integration mode
(TDI-mode) operation can be performed during an extended-time
illumination pulse. During this TDI-mode operation, charges stored
by pixels of the image sensor are shifted only in a first
direction, and track the image motion. Notably, a split-readout
operation is performed only during non-illumination. During this
split-readout operation, first charges stored by first pixels of
the image sensor are shifted in the first direction and second
charges stored by second pixels of the image sensor are
concurrently shifted in a second direction, the second direction
being opposite to the first direction.
Inventors: |
Brown; David L.; (Los Gatos,
CA) ; Chuang; Yung-Ho; (Cupertino, CA) ;
Yuditsky; Yury; (Mountain View, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KLA-Tencor Corporation |
Milpitas |
CA |
US |
|
|
Assignee: |
KLA-Tencor Corporation
Milpitas
CA
|
Family ID: |
50879908 |
Appl. No.: |
14/096911 |
Filed: |
December 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61735427 |
Dec 10, 2012 |
|
|
|
Current U.S.
Class: |
250/208.1 |
Current CPC
Class: |
G01N 2021/95676
20130101; G01N 2021/8896 20130101; H04N 5/3743 20130101; H04N
5/3742 20130101; H04N 5/2256 20130101; G01N 21/8851 20130101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H04N 3/14 20060101
H04N003/14 |
Claims
1. A method of operating an image sensor with a continuously moving
object, the method comprising: performing a timed delay integration
mode (TDI-mode) operation during an illumination pulse, wherein
charges stored by pixels of the image sensor are shifted only in a
first direction during TDI-mode operation; and performing a
split-readout operation during non-illumination, wherein first
charges stored by first pixels of the image sensor are shifted in
the first direction and second charges stored by second pixels of
the image sensor are concurrently shifted in a second direction
during the split-readout operation, the second direction being
opposite to the first direction.
2. The method of claim 1, wherein the TDI-mode operation is
synchronized with the illumination pulse.
3. The method of claim 1, wherein the TDI-mode operation is
triggered to start within one clock period of the illumination
pulse using electronic or optical synchronization.
4. The method of claim 1, wherein a time of the TDI-mode operation
includes a period of the pulsed illumination.
5. The method of claim 1, wherein during the split-readout
operation, the image sensor is not synchronized with an image
motion.
6. The method of claim 1, wherein performing the split-readout
operation includes a parallel readout of a plurality of serial
registers.
7. The method of claim 1, further including: providing an idle
operation between the TDI-mode operation and the split-readout
operation.
8. The method of claim 1, providing the idle operation is performed
before the TDI-mode operation of the image sensor.
9. The method of claim 1, wherein an illumination interval includes
a plurality of illumination pulses extending over one or more TDI
line periods.
10. The method of claim 9, further including recovering from a
pixel defect of the image sensor based on analyzing pixel outputs
corresponding to the plurality of illumination pulses.
11. A system comprising: a pulsed illumination source; an image
sensor; optical components configured to direct pulsed illumination
from the pulsed illumination source to a continuously moving
object, and direct reflected light from the object to the image
sensor; and a processor configured to operate the image sensor, a
configuration performing a process comprising: performing a timed
delay integration mode (TDI-mode) operation during an illumination
pulse, wherein charges stored by pixels of the image sensor are
shifted only in a first direction during TDI-mode operation; and
performing a split-readout operation during non-illumination,
wherein first charges stored by first pixels of the image sensor
are shifted in the first direction and second charges stored by
second pixels of the image sensor are concurrently shifted in a
second direction during the split-readout operation, the second
direction being opposite to the first direction.
12. The system of claim 11, wherein the TDI-mode operation is
synchronized with the illumination pulse.
13. The system of claim 11, wherein the TDI-mode operation is
triggered to start within one clock period of the illumination
pulse using electronic or optical synchronization.
14. The system of claim 11, wherein a time of the TDI-mode
operation includes a period of the pulsed illumination.
15. The system of claim 11, wherein during the split-readout
operation, the image sensor is not synchronized with an image
motion.
16. The system of claim 11, wherein performing the split-readout
operation includes a parallel readout of a plurality of serial
registers.
17. The system of claim 11, further including: providing an idle
operation between the TDI-mode operation and the split-readout
operation.
18. The system of claim 11, providing the idle operation is
performed before TDI-mode operation of the image sensor.
19. The system of claim 11, wherein an illumination interval
includes a plurality of illumination pulses extending over one or
more TDI line periods.
20. The system of claim 19, further including recovering from a
pixel defect of the image sensor based on analyzing pixel outputs
corresponding to the plurality of illumination pulses.
Description
RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional Patent
Application 61/735,427, entitled "Method And Apparatus For High
Speed Acquisition Of Moving Images Using Pulsed Illumination" filed
Dec. 10, 2012.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to systems configured to use both
timed delay integration and pulsed illumination while owing
high-speed image scanning.
[0004] 2. Related Art
[0005] Time delay integration (TDI) is an imaging process that
produces a continuous image of a moving object that can be much
larger than the field of view of the imaging hardware. In a TDI
system, image photons are converted to photocharges in a sensor
comprising an array of pixels. As the object is moved, the
photocharges are shifted from pixel to pixel down the sensor,
parallel to the axis of movement. By synchronizing the photocharge
shift rate with the velocity of the object, the TDI can integrate
signal intensity at a fixed position on the moving object to
generate the image. The total integration time can be regulated by
changing the speed of the image motion and providing more/less
pixels in the direction of the movement. In conventional TDI
inspection systems, the readout circuits are positioned on one side
of the sensor to read out the integrated signal. TDI inspection
systems can be used for inspecting wafers, masks, and/or
reticles.
[0006] In a system with continuous illumination and a moving
object, the TDI must be precisely synchronized to the image motion
so that the recorded image is not blurred. One disadvantage of this
system is that the readout of the sensor can be in only one
direction, i.e. in the direction corresponding to the image motion,
and must operate at the same scan rate as the object during the
illumination pulse. In a system with pulsed illumination and a
moving object, the image can be collected almost instantly over the
entire sensor area. The image can then be read out along both sides
of the sensor, thereby effectively doubling the readout speed. The
readout line rate can also be faster than the image scan rate
without compromising the final image quality, which can further
increase readout speed. A critical disadvantage of this system is
that the illumination pulse must be very short so that the moving
image does not produce blur during the exposure time. As the pulsed
illumination time approaches the sensor line period, the image
motion will start to cause significant blur, and the image will
degrade severely beyond that threshold. Another disadvantage of
this system using very short pulses is that the image information
at defective pixel locations on the sensor cannot be recovered.
[0007] Therefore, a need arises for a method and apparatus that
provides a continuously moving object, pulsed illumination, fast
readout capability, and recovery of image information where sensor
pixels are defective.
SUMMARY
[0008] A method of operating an image sensor with a continuously
moving object is described. In this method, a timed delay
integration mode (TDI-mode) operation can be performed during an
extended-time illumination pulse. During this TDI-mode operation,
all charges stored by pixels of the image sensor are shifted only
in a first direction, and track the image motion. Notably, a
split-readout operation is performed only during non-illumination.
During this split-readout operation, first charges stored by first
pixels of the image sensor are shifted in the first direction and
second charges stored by second pixels of the image sensor are
concurrently shifted in a second direction, the second direction
being opposite to the first direction.
[0009] The TDI-mode operation is synchronized with the illumination
pulse. In one embodiment, the TDI-mode operation is triggered to
start within one clock period of the illumination pulse using
electronic or optical sychronization. The time of the TDI-mode
operation includes a period of the pulsed illumination. During the
split-readout operation, the image sensor charge movement is not
synchronized with the image motion. In one embodiment, performing
the split-readout operation can include a parallel readout of a
plurality of sensor output channels.
[0010] An idle operation can be provided before the TDI-mode
operation and the split-readout operation (and in one embodiment,
also between the TDI-mode operation and the split-readout
operation) in order to facilitate synchronization of object and
sensor readout, or to reduce power consumption of the detection
system. In one embodiment, an illumination interval can include a
plurality of illumination pulses. Analyzing the pixel outputs
corresponding to the plurality of illumination pulses that extend
over one or more TDI line periods can improve the image quality
near pixel defects on the image sensor.
[0011] A system for inspection or metrology is also described. This
system includes a pulsed illumination source, an image sensor,
optical components, and a processor. The illumination pulse may be
similar to or longer than the line period of the sensor. The
optical components are configured to direct pulsed illumination
from the pulsed illumination source to an object, and direct
reflected light from the object to the image sensor. The processor
is configured to operate the image sensor. A configuration includes
performing a process including the TDI-mode operation and the
split-readout operation, as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates an exemplary scanning inspection system
using pulsed illumination with a continuously moving object.
[0013] FIG. 2A illustrates an exemplary image sensor having two
sides, which can be operated independently.
[0014] FIG. 2B illustrates the operation of exemplary CCD gates,
which can be used for the image sensor.
[0015] FIG. 3A illustrates an exemplary timing diagram, with three
distinct operating modes, for a three-phase CCD in a system with
pulsed illumination.
[0016] FIG. 3B illustrates how charge is shifted in different
directions in the sensor image collection and storage region, based
on the sequence of the CCD drive signals.
[0017] FIG. 4 illustrates exemplary drive signals and relative
timing for a three-phase CCD.
DETAILED DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 illustrates an exemplary system 100 configured to use
a pulsed illumination source 106 with a continuously moving object
101, such as a wafer, mask, or reticle. Advantageously, pulsed
illumination 106 can be a long pulse. Exemplary sources for pulsed
illumination 106 can include a Q-switched laser or a pulsed lamp. A
Q-switched laser uses a variable attenuator inside the laser's
optical resonator to produce light pulses with extremely high peak
power. These light pulses are much higher than those produced by
the same laser operating in continuous mode. A pulsed lamp could be
implemented by a deep ultraviolet (DUV) excimer or an extreme
ultraviolet (EUV) source. In one preferred embodiment, the pulse
duration is close to or longer than the line period of the TDI. For
a line period of 1 microsecond, suitable illumination could be near
500 ns, or beyond 10's or even 100's of microseconds, with
significant benefit from the described method of this
invention.
[0019] In system 100, a beam splitter 107 would direct illumination
pulses from pulsed illumination source 106 to an objective lens
104, which would focus that light onto object 101. Reflected light
from object 101 would then be directed to an image sensor 110. Note
that other well-known optical components for directing and focusing
of the light are not shown for simplicity in FIG. 1. For example,
U.S. Pat. No. 5,717,518, which issued Feb. 10, 1998, and U.S.
patent application Ser. No. 13/554,954, which was filed Jul. 9,
2012, both of which are incorporated by reference herein, describe
exemplary optical components that can be used in system 100. A
processor 120, which is coupled to image sensor 110, is configured
to provide synchronization of illumination pulses from pulsed
illumination source 106 with control and data signals to and from
image sensor 110 as well as analysis of the image data (described
in detail below). In the above-described configuration, object 101
has an object motion 103 and image sensor 110 has an image motion
109.
[0020] In accordance with one aspect of system 100, because of
object motion 103, the illuminated region will continuously move
across object 101 as indicated by illuminated region 102a (e.g.
time period N), previously illuminated region 102b (e.g. time
period N-1), and previously illuminated region 102c (e.g. time
period N-2). Each of illuminated regions 102a, 102b, and 102c can
be a thin rectangular-shaped region (not shown to scale for ease of
viewing). Note the regions are shown separated for clarity, but may
overlap to provide 100% imaging coverage, or for additional
redundancy and performance during defect detection.
[0021] FIG. 2A illustrates an exemplary split-readout image sensor
110 including two sets of readout circuits 201A and 201B positioned
on either side of an image region 203. Readout circuits 201A and
201B can include serial registers 202A and 202B and readout
amplifiers 204A and 204B, as well as other components such as
transfer gates. Exemplary embodiments of readout circuits 201A and
201B, as well as other components of sensor 110 are described in
U.S. Pat. No. 7,609,309, entitled "Continuous Clocking of TDI
Sensors", issued Oct. 27, 2009, which is incorporated by reference
herein. Image region 203 is a two-dimensional (2D) array of pixels,
and each line of the image is read out concurrently in each
direction A and B. Each line is then read out one pixel at a time
in the simplest case. Therefore, in preferred embodiments, the
serial registers 202A and 202B can be divided into a plurality of
register segments (e.g. FIG. 2A shows each serial register being
divided into six segments, thereby allowing parallel read out using
a plurality of amplifiers 204A and 204B.
[0022] Notably, readout circuits 201A and 201B can be operated
independently, thereby allowing image sensor 110 to provide two
readout directions A and B. In a split-readout mode, each side of
image region 203 (i.e. sides 203A and 203B) can be synchronously
clocked to read out one image line into their respective output
channels. In one embodiment, image region 203 may have 1000 lines,
each line formed by a column of pixels. Therefore, during the
split-readout mode, 500 lines could be read out in direction A and,
concurrently, 500 lines could be read out in direction B.
[0023] This split-readout mode is possible based on the timed
activation of the charge-coupled device (CCD) drivers in image
sensor 110. For example, a plurality of CCD drivers P1a, P2a, P3a,
P1b, P2b, and P3b can be used to provide phases. As shown in FIG.
2B, CCD drivers P1a, P2a, P3a, P1b, P2b, and P3b can be
characterized as driving sets of gate electrodes (hereinafter
gates), each set having six gates. In one preferred embodiment of
image sensor 110, three gates are provided for each pixel to
provide three phases. In FIG. 2B, two pixels 210 and 211 are shown,
wherein gates 231, 232, and 233 are positioned over pixel 210 and
gates 234, 235, and 236 are positioned over pixel 211. In image
sensor 110, pixels 210 and 211 are aligned along the read-out axis
to form part of a column of the 2D array of pixels forming image
region 203.
[0024] Image region 203 can be implemented as an optical sensor or
a photocathode. In one optical sensor embodiment, image region 203
can include a photo-sensitive p-type silicon substrate 214 and an
n-type buried channel 213. The electrostatic forces in silicon
substrate 214 are determined by the voltage level applied to a
particular gate by a clock input signal (e.g. one of clock signals
from CCD drivers P1a, P2a, P3a, P1b, P2b, and P3b). High level
voltages induce the formation of a potential "well" beneath the
gate, whereas low level voltages form a potential barrier to
electron movement. To ensure that charge from one pixel is not
mixed with other pixels, a gate voltage is driven high when an
adjacent gate voltage is driven low (described in further detail in
reference to FIGS. 3A and 3B). At an initial state at time 220,
gates 231 and 234 of pixels 210 and 211, respectively, have high
level voltages that form potential wells with integrated charge
(i.e. electrons), and gates 232, 233 (of pixel 210) and 235, 236
(of pixel 211) have low level voltages that form potential
barriers. At a subsequent time 221, gates 232 and 235 of pixels 210
and 211, respectively, have high level voltages that form potential
wells with integrated charge (i.e. electrons), and gates 231, 233
(of pixel 210) and 234, 236 (of pixel 211) have low level voltages
that form potential barriers. At yet a subsequent time 222, gates
233 and 236 of pixels 210 and 211, respectively, have high level
voltages that form potential wells with integrated charge (i.e.
electrons), and gates 231, 232 (of pixel 210) and 234, 235 (of
pixel 211) have low level voltages that form potential barriers.
Note that adjacent gates when shifting charge preferably both have
a high level voltage for a short time to facilitate charge
transfer. (FIG. 3A, which is described below) shows this timing
overlap.) Thus from time 220 to time 222, the charge is shifted
from left to right, i.e. from pixel 210 to pixel 211. This
directional shifting of charge can be advantageously modified
during modes of the inspection system, as described in reference to
FIG. 3.
[0025] FIG. 3A illustrates an exemplary timing diagram 300
indicating signals output by CCD drivers P1a, P2a, P3a, P1b, P2b,
and P3b, clock signals (ck), an external synchronization pulse
(sync), and a pulsed illumination time (pulse). Note that the start
and stop of a voltage transition of each signal output by the CCD
drivers can be synchronized to the clock signals ck. The external
synchronization pulse sync triggers a three-mode cycle (one
complete cycle being shown in FIG. 3A). In the example of FIG. 3A,
one laser pulse is provided during each cycle.
[0026] The three sensor modes are indicated in FIG. 3A as "0", "1",
and "2". Sensor mode 1 is TDI-mode operation in which the laser
pulse occurs and therefore an image of the illuminated region of
the object can be generated. In one embodiment, the pulse duration
may be close to or longer than the line period of the high-speed
TDI, which could be, for example, 1 microsecond. Note that because
the illumination pulse can be long (e.g. more than 1 microsecond),
a fixed point on the image will shift across one or more sensor
pixels. Therefore, consecutive clocking of CCD drivers P1a/P1b,
P2a/P2b, and P3a/P3b (shown in FIG. 3) can be performed to ensure
that the generated charge in the image region is shifted along with
the image, to provide TDI-mode operation and ensure no blurring. In
some embodiments, shifting of charge may be performed between just
1-2 pixels to ensure no blurring of the image occurs. The rate of
this charge shifting, also called the sensor line rate, can be
chosen to accurately match the motion of the image. The total time
in the TDI mode of operation may be just one or a few line clock
periods, depending on the total illumination pulse time. However,
image quality loss due to blurring and the resulting degradation of
defect detection would be very significant without providing the
TDI-mode of operation.
[0027] Sensor mode 2 is high-speed split-readout operation in which
illumination is off (i.e. no laser pulse is present). Notably,
because the illumination is off, data can be read out from two
sides (e.g. sides 203A and 203B of image region 203, FIG. 2A) as
fast as the clock signals for the serial registers will allow.
During this time, image sensor 110 is not synchronized with image
motion 109.
[0028] Referring back to FIG. 2A, in one embodiment, an actual
illumination region 205 may be slightly smaller than the sensor
image region 203. Therefore, when charge shifting occurs during
TDI-mode operation, the image will move outside the optical field
of view. However, the image is still stored by image region 203
because of the charge stored in the pixels. Therefore, during the
high-speed split-readout mode, there would be some blank or
lower-signal lines that are first read out before uniformly
illuminated image data. This artifact can be compensated for during
processing, or ignored if a suitable image frame overlap is chosen
that allows for redundancy near the frame edges. Specifically, when
the signals are output from the amplifiers, the image can be
reconstructed with compensation for illumination effects near the
edge of the image frame.
[0029] Sensor mode 0 is idle operation in which the sensor image
charge (not the object) is static (i.e. stopped). In one
embodiment, one set of signals, such as signals of CCD drivers P3a
and P3b, can be kept high during the idle operation to provide
pre-charging of predetermined pixels and ensure transition between
states without losing signal charge or image data at the edge of
field. Note that the image sensor needs to rapidly measure charge
on each pixel with an accuracy of less than one millivolt. The
image sensor may not be able to make such a measurement in the
presence of voltage noise in the substrate. To address this issue,
moving the charge from pixel to pixel can be discontinued while the
readout amplifiers read signals from the serial registers. In one
embodiment, the illumination pulse can occur after a period
sufficient for at least one charge transfer to occur due to timing
uncertainties of the illumination source. The trigger for the
sensor to begin TDI-mode operation can be derived from the camera
clock or based on optical detection of the illumination pulse.
Since the object motion and image sensor line rate are well
synchronized, the timing stability of the source can be quite poor
and yet allow for a sharp and accurately positioned image to
result. After the end of sensor mode 2, image sensor 110 returns to
sensor mode 0 (idle mode), and waits for the next synchronization
signal and illumination pulse. Note that the processing, buffering,
and transport of the collected data from amplifiers 204A and 204B
to an external image processing computer (not shown) may proceed
during all sensor modes.
[0030] FIG. 3B illustrates how charge is shifted in different
directions based on the sequence of the CCD drive signals.
Specifically, during TDI-mode operation (sensor mode 1) 320, the
CCD drive signals are sequenced so that charges can be shifted
through the pixels all in one direction. In contrast, during
split-readout operation (sensor mode 2) 321, the CCD drive signals
are sequenced so that the half of the charges are shifted in one
direction and the other half of the pixel's charges are shifted in
the opposite direction. Note that each CCD drive signal is provided
to all the gates in one or more pixel columns of the image region
of the sensor array. Thus, the sequence is based on the physical
wiring of the sensor. Although 18 columns are shown in FIG. 3B,
other embodiments of a sensor array can include fewer or more
columns of pixels.
[0031] In some embodiments, the COD drive signals may be square, as
shown in FIG. 3A. In other embodiments, the CCD drive signals may
have other shapes. For example, FIG. 4 illustrates sinusoidal drive
signals for a three-phase CCD. For example, voltage waveforms 401,
402, and 403 can respectively drive gates 231 and 234, gates 232
and 235, and gates 233 and 236 in image region 203 (see also, FIG.
2A). Notably, these waveform shapes operate at different voltage
phases in adjacent gates in such a way to provide a substantially
de minimus net voltage fluctuation on ground and DC voltage
reference planes, thereby reducing noise. Moreover, transferring
charge using a non-square waveform, e.g. sinusoidal, rather than a
square waveform generally requires lower peak currents to control
the gates. As a result, the peak displacement currents flowing in
the substrate are much lower, thereby ensuring lower voltage
fluctuations and reduced heat generation in the substrate.
[0032] The low levels of voltage fluctuations in the substrate also
enable the system to accurately read out the contents of the pixels
in the serial register with sufficient sensitivity even when the
sensor is transferring charge in the image region. Thus, by using
the sinusoidal waveforms, the readout amplifiers may concurrently
operate while moving the charge in the image area from one pixel to
another. In other embodiments, the CCD drive signals may have other
non-square waveform shapes, which can also provide similar benefits
to those discussed for sinusoidal waveforms.
[0033] In one embodiment, instead of having one pulse per sensor
cycle (i.e. sensor modes 0, 1, 2), multiple, grouped pulses (e.g. a
strobe-like set of at least two pulses) can be used. After readout,
the image can be processed to take into account the multiple,
grouped pulses. Specifically, because the location of the object
and the illumination timing is known, the measured image can be
deconvolved into a corrected "true" image. This type of pulsing and
subsequent processing can improve the sensitivity of the original
image because at least two samples are provided for each pixel.
Specifically, the multiple pulses provide a higher signal to noise
ratio because at least twice as much data is provided (compared to
a single pulse) with little additional noise (as described
above).
[0034] Moreover, having two samples for each pixel may be
beneficial when a predominantly dark image with a few bright spots
(indicating defects, for example) is captured. In this type of
image, there is minimal interference with other parts of the image.
Two bright spots generated during readout can be deconvolved to
determine whether, for example, two bright spots are in fact one
defect. This deconvolving would use information including time and
image movement speed (because image would move between pulses) to
provide the requisite reconstruction.
[0035] Note that this multiple, grouped pulse embodiment may also
be used to address sensor defect detection. Specifically, if there
is a defect on the sensor itself (which would result in missing
information on the image), then two samples allow for the inclusion
of image information that would otherwise be unavailable. In other
words, images over more than one pixel can be collected during the
illumination pulses to recover image data at a defective sensor
pixel location. This multiple pulse operation could reduce the cost
of sensors because increased levels of imperfections can be allowed
in sensors while still ensuring the collection of all image
information (or substantially all image information in the unlikely
event that the two sensor defects coincide with the pixels
capturing image data during the multiple, grouped pulses).
[0036] As described above, system 100 can advantageously combine
certain beneficial properties of TDI readout mode with fast readout
capability of pulsed image architectures. Because of the fast
readout speed, system 100 can effectively reduce cost of ownership.
In addition, system 100 provides for improved resolution of the
image for illumination times that would blur when collected with
conventional image sensors. In other words, conventional image
sensors cannot use long pulse light sources due to image blurring.
Notably, long pulse light sources can reduce wafer damage by
reducing peak power illumination. Moreover, system 100 can use
various CCD drive waveform shapes, including sinusoidal waveforms.
These sinusoidal waveforms can be used effectively in high-speed
inspection and metrology applications where low noise is critical.
In addition, the continuous-clocking technique (i.e. the three
sensor modes described above, with the idle mode using fixed
voltages in both square-wave and sinusoidal waveform operation) can
reduce heat generation and mitigate the negative effects of timing
jitter in the control and readout electronics.
[0037] Although illustrative embodiments of the invention have been
described in detail herein with reference to the accompanying
figures, it is to be understood that the invention is not limited
to those precise embodiments. They are not intended to be
exhaustive or to limit the invention to the precise forms
disclosed. As such, many modifications and variations will be
apparent to practitioners skilled in this art. For example, in one
embodiment, the image sensor could comprise a back-illuminated
back-thinned CCD. Back-illuminating a thinned sensor ensures good
sensitivity to UV light. In some embodiments, the back-illuminated
back-thinned CCD could have a thin boron coating on its
light-sensitive back surface in order to increase the lifetime of
the device when used with DUV or vacuum UV radiation. The use of
boron coatings on back-thinned sensors is described in U.S.
Provisional Patent Application 61/622,295 by Chern et al, filed on
Apr. 10, 2012. This provisional application is incorporated by
reference herein. In some embodiments, the image sensor could
comprise an electron-bombarded CCD (EBCCD) sensor. EBCCDs have high
sensitivity and low noise for very low light levels as are often
encountered in dark-field inspection systems. In some embodiments
of the EBCCD, the CCD may be a back-thinned device with a boron
coating on its back surface in order to improve the sensitivity of
the CCD to low-energy electrons and, hence, improve the image
sensor noise and spatial resolution. The use of boron coatings in
EBCCDs is described in U.S. Provisional Patent Application
61/658,758 by Chuang et al, filed on Jun. 12, 2012, which is also
incorporated by reference herein. Accordingly, it is intended that
the scope of the invention be defined by the following claims and
their equivalents.
* * * * *